Description:
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Opera browser a support Web Forms 2.0, it allows easy build uploading
multiple files form, but current PHP version not meet the standard
RFC2388 www.ietf.org/rfc/rfc2388.txt
Returning Values from Forms: multipart/form-data, L. Masinter. IETF, August 1998.
4.2 Sets of files
If the value of a form field is a set of files rather than a single
file, that value can be transferred together using the
?multipart/mixed? format.
Therefore, at present, using PHP, it is impossible to ensure efficiency uploading multiple files form at Opera browser.
Reproduce code:
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<html>
<body>
<form action="upload.php" enctype="multipart/form-data" method="post">
<input type="file" min="1" max="9999" name="file[]" multiple="true"/>
<input type="submit" name="submit" value="Upload"/>
</form>
</body>
</html>

History

Per bug #47789, this also affects HTML forms with
<input type="file" multiple> elements in certain browsers, most notably
Opera. Interestingly, Firefox and Chrome don't use multipart/mixed,
probably to avoid this sort of issue, but according to the spec this
should be supported.